Polyester film and use thereof

Abstract

In order to provide a foldable display that is suitable for mass production and that has no risk of image distortion at its folding portion after the display is repeatedly folded, the invention provides a polyester film for foldable displays in which no cracks occurs in the folding portion. In particular, the invention provides a polyester film for a foldable display, the film having a thickness of 10-125 μm and a high-temperature hold angle in a bending direction of 70° or more, wherein the high-temperature hold angle refers to an angle of a crease formed after fixing the film at a heating temperature of 85° C. for 18 hours in such a manner that a strain of 1.7% is applied to both surfaces of a bent portion of the film, and the bending direction refers to a direction that is orthogonal to a folding portion.

The invention claimed is: 
     
         1 . A polyester film for a foldable display, wherein the polyester film is a single-layer film comprising 95 weight % or more of a polyethylene-2,6-naphthalene dicarboxylate homopolymer satisfying the following conditions:
 (1) having a refractive index in a bending direction of 1.644 to 1.730, and   (2) having a refractive index in a folding direction of 1.770 to 1.854, and   (3) having a thickness of 10 μm or more and 125 μm or less and a high-temperature hold angle in a bending direction of 70° or more, wherein the high-temperature hold angle refers to an angle of a crease formed after fixing the polyester film at a heating temperature of 85° C. for 18 hours in such a manner that a strain of 1.7% is applied to both surfaces of the polyester film at a bent portion, and the bending direction refers to a direction that is orthogonal to a folding portion.   
     
     
         2 . The polyester film for a foldable display according to  claim 1 , having a density of 1.349 g/cm 3  or more. 
     
     
         3 . The polyester film for a foldable display according to  claim 2 , comprising an easy-to-adhere layer on at least one surface of the polyester film. 
     
     
         4 . A foldable display comprising the polyester film for a foldable display of  claim 3 , the polyester film being disposed as a back protection film, wherein the polyester film is a single continuous polyester film placed through the folding portion of the foldable display. 
     
     
         5 . A mobile device comprising the foldable display of  claim 4 . 
     
     
         6 . A foldable display comprising the polyester film for a foldable display of  claim 2 , the polyester film being disposed as a back protection film, wherein the polyester film is a single continuous polyester film placed through the folding portion of the foldable display. 
     
     
         7 . A mobile device comprising the foldable display of  claim 6 . 
     
     
         8 . The polyester film for a foldable display according to  claim 1 , comprising an easy-to-adhere layer on at least one surface of the polyester film. 
     
     
         9 . A foldable display comprising the polyester film for a foldable display of  claim 8 , the polyester film being disposed as a back protection film, wherein the polyester film is a single continuous polyester film placed through the folding portion of the foldable display. 
     
     
         10 . A mobile device comprising the foldable display of  claim 9 . 
     
     
         11 . A foldable display comprising the polyester film for a foldable display of  claim 1 , the polyester film being disposed as a back protection film, wherein the polyester film is a single continuous polyester film placed through the folding portion of the foldable display. 
     
     
         12 . A mobile device comprising the foldable display of  claim 11 .